Video Transcript
A student is setting up an
experiment to measure the enthalpy change in a neutralization reaction between
hydrochloric acid and potassium hydroxide. Before mixing the two solutions
together in a polystyrene cup, the student places a beaker of each solution in a
water bath set to 25 degrees Celsius. Why does the student do this? (A) To increase the rate of
reaction between the hydrochloric acid and the potassium hydroxide. (B) To remove any impurities in the
solutions. (C) To ensure the reactants have
the correct activation energy needed to react. (D) To ensure both solutions are at
the same temperature as each other. (E) To make the solutions less
viscous.
The question describes an
experiment used to measure the enthalpy change of the neutralization reaction
between hydrochloric acid and potassium hydroxide. This type of experiment is a
calorimetry experiment. A calorimetry experiment can be
used to measure the heat energy change or enthalpy change taking place in a
reaction. To perform a calorimetry
experiment, we need a calorimeter, an insulated reaction vessel. We can construct a basic
calorimeter by simply using a polystyrene cup with a lid. The solutions being reacted are
added to the calorimeter, and the temperature is monitored with a thermometer.
When the reaction takes place, heat
energy is either absorbed or released. This results in a change in
temperature. We can then relate the observed
change in temperature to the change in heat or the enthalpy change. Because we use the change in
temperature to determine the enthalpy change, it’s important that this temperature
change is only due to the reaction taking place. We know that if we mix a hot
solution with a cold solution, the temperature will change. This temperature change is due to
the transfer of kinetic energy from the molecules in the hot solution to the
molecules in the cold solution.
In this question, the goal of the
experiment was to measure the enthalpy change of a reaction. If the temperature of the two
solutions used in the experiment is different, then the temperature change is due
not only to the reaction, but also the kinetic energy transfer between the different
temperature solutions. This is why the student places a
beaker of each of the solutions in a water bath before performing the
experiment. By doing this, the student ensures
that each of the solutions is at the same temperature and that the resulting
temperature change when the solutions are combined is only due to the heat absorbed
or released during the reaction.
Therefore, the reason why the
student places a beaker of each solution in a water bath before mixing the solutions
together is answer choice (D): to ensure both solutions are at the same temperature
as each other.
